This invention relates to a method of fracturing subsurface formations in the vicinity of a borehole.
Fracturing of oil and gas well formations has been successful for the purpose of increasing the flow of petroleum fluids. Such fracturing can be effected by injecting a liquid into the well under sufficient pressure to fracture the formation. This technique is known as hydraulic fracturing. According to another technique, fracturing can be effected by the use of explosives such as ammonium nitrate. Such fracturing techniques as discussed above act to form fissures in the subsurface formation around the well borehole so as to increase the permeability of the formation, thus enhancing oil and/or gas flow from the formation into the borehole. Fractured formations may be further stimulated by forcing a propping agent into the fractures or fissures under pressure. If the propping agent is of sufficient size and strength it has the ability to hold the fractures open so that oil and gas may pass more freely from the formation into the borehole.
According to one particular prior fracturing method disclosed in U.S. Pat. No. 3,713,487 of Lozanski, explosives and propping agents are employed in combination to stimulate the flow of petroleum fluids in a well. More specifically, a high explosive, such as dynamite, together with a propping agent, such as glass beads, are placed in a container provided with a detonator. The container is lowered in the well borehole to a zone where fracturing is desired, and the void space remaining between the container and the borehole walls is filled with a high explosive such as ammonium nitrate. The explosive in the container is then detonated so as to cause fracturing in the subsurface formation around the borehole, the force of the explosion simultaneously forcing propping agent into the fissures thus produced.
High explosives such as dynamite used in the above described method generate an extremely high velocity shock wave upon detonation. This wave generally propagates at a velocity of several thousand meters per second. Such high velocity shock waves cause appreciable damage to rock formations around the borehole. Such damage typically involves packing of rock around the borehole so as to reduce permeability and thus also reduce flow of oil or gas into the borehole.